There has heretofore been known an electrostatic printing apparatus for attaching powdery ink onto a surface of an object by using an electrostatic force to print a printed pattern including characters and figures on the surface of the object. FIG. 7 is a schematic diagram showing an arrangement of this type of electrostatic printing apparatus. The conventional electrostatic printing apparatus has a stencil screen 110 disposed above an object 100, a rotation brush 120 on the screen 110, and a hopper 140 for supplying powdery ink 130 onto the brush 120. A printed pattern including characters and figures is formed of a mesh 111 on the screen. The rotation brush 120 uses a soft open-cell urethane sponge in view of its good rubbing characteristics for rubbing of ink into the screen 110.
The powdery ink 130 supplied from the hopper 140 is pushed out downwardly through the mesh 111 of the screen 110 by rotation of the brush 120. A high direct-current voltage is applied between the object 100 and the screen 110 by a direct-current power supply DC to form an electrostatic field between the object 100 and the screen 110. The powdery ink which has passed through the mesh 111 and has thus been charged travels straight toward the object 100, which serves as a counter electrode, in the electrostatic field and is attached to a surface of the object 100. Thus, a printed pattern in the screen 110 which includes characters and figures is printed on the surface of the object 100.
When the powdery ink is rubbed into the screen 110, the powdery ink is heated due to pressure applied by the brush 120 and frictional heat. Therefore, if a printing process is continuously performed with powdery ink containing oils and fats, for example, then the temperature of the powdery ink may be increased to not less than the melting points of the oils and fats, and the powdery ink may be melted. When the melted ink is cooled, it solidifies on a surface of the screen 110, thereby causing clogging of the mesh 111 formed in the screen 110. Accordingly, with the conventional electrostatic printing apparatus, it is necessary to interrupt the printing process to perform a cleaning process of the screen 110 in order to prevent the clogging of the mesh 111, and thus the apparatus cannot be operated continuously.
Further, according to the printed pattern in the screen 110, the consumption of the powdery ink may be different from one location to another on the screen 110. In such a case, there is a large tendency that the amount of the powdery ink on the screen 110 is reduced at locations where the consumption of the powdery ink is large, while there is a large tendency that the powdery ink is accumulated on the screen 110 at locations where the consumption of the powdery ink is small. In this manner, since the distribution of the amount of ink is not uniform at locations on the screen 110, uniform and clean printing cannot be achieved in some cases.
Here, if the amount of ink 130 to be supplied from the hopper 140 is adjusted according to the consumption of ink, then non-uniform printing described above can be solved. However, it is difficult to vary the amount of ink 130 to be supplied from the hopper 140 according to locations on the screen 110. Furthermore, even if the amount of ink 130 to be supplied can be adjusted according to the consumption of ink, the amount of ink to be supplied has to be readjusted each time the printed pattern in the screen 110 is changed, thereby causing considerably troublesome work.
Furthermore, a portion of the powdery ink that has passed through the mesh 111 may not travel toward the object 100 and may be attached to a lower surface of the screen 110. If the ink is attached to the lower surface of the screen 110, then the mesh 111 is gradually clogged with the ink on the lower surface of the screen 110, and thus the ink is unlikely to be pushed out through the mesh 111 so as to cause defective printing. As a result, a cleaning device 150 shown in FIG. 8 has heretofore been provided to remove ink attached to the lower surface of the screen 110. As shown in FIG. 8, the conventional cleaning device 150 has a suction nozzle 151 and rubber pieces 152 which are brought into contact with the lower surface of the screen 110, and draws the ink attached to the lower surface of the screen 110 from the suction nozzle 151.
When such a cleaning device 150 is employed, strong suction force is required to draw ink attached to the lower surface of the screen 110. When the suction force of the cleaning device 150 is large, ink near the mesh 111 on an upper surface of the screen 110 may unnecessarily be drawn and removed. If the amount of ink near the mesh 111 is reduced, then the distribution of ink on the screen 110 becomes non-uniform so as to produce light and shade in a printed object. Additionally, the ink excessively drawn from the upper surface of the screen 110 results in a loss.